Diaquabis ( L-serinato ) copper ( II ) 0 . 1-hydrate at 120 K

# 2005 International Union of Crystallography Printed in Great Britain – all rights reserved The title compound, [Cu(C3H6NO3)2(H2O)2] 0.1H2O, is isostructural with the nickel analogue. The octahedral Cu ion lies on a twofold axis, with cis chelating O,N-serine groups and trans aqua ligands. Small amounts of a solvent water molecule form hydrogen bonds to link the molecules along the [010] direction, while a number of strong hydrogen bonds combine to form sheets in the (110) plane.

The title compound, [Cu(C 3 H 6 NO 3 ) 2 (H 2 O) 2 ]Á0.1H 2 O, is isostructural with the nickel analogue. The octahedral Cu II ion lies on a twofold axis, with cis chelating O,N-serine groups and trans aqua ligands. Small amounts of a solvent water molecule form hydrogen bonds to link the molecules along the [010] direction, while a number of strong hydrogen bonds combine to form sheets in the (110) plane.
Complex (1), isostructural with the analogous nickel complex, diaquabis(l-serinato)nickel(II) hydrate, (3), (van der Helm & Hossain, 1969), has an octahedrally coordinated Cu II ion with cis chelating O,N-ser groups and trans aqua ligands (Fig. 1). A similar cis arrangement of ser units arises in square-pyramidal (2), in which a carboxylate O atom, from an adjacent molecule, occupies the apical position. A distant O atom is sited 3.632 (6) Å from Cu trans to the apical ligand in (2), but this can at most be considered only a very weak interaction. Comparison of the serine-Cu bond lengths in (2) [Cu-O 1.952 (5) and 1.970 (5) Å ; Cu-N 1.975 (6) and 1.988 (6) Å ] and in (1) ( Table 1) indicates that the weaker interactions occur in the higher coordinate complex, (1). The serine chelate rings in (1) have envelope conformations with flaps at the N atoms. The Cu II ion and the four serine binding atoms are essentially co-planar.
Small amounts of additional water molecules are present in both (1) and (3). The space group and structure of (1) are notably different from those of the unhydrated compound, (2), and although only a very small amount of water was found to be present in (1), both the hydrogen-bonding scheme (see below) and the availability of space (PLATON; Spek, 2003) confirm its presence.
The solvent water molecule forms hydrogen bonds (Table 2) with the O atom of the aqua ligand in the main molecule ( Fig. 2), leading to chains along [010]. Together with the other strong hydrogen bonds (Table 2), these form sheets in the (110) plane (Fig. 2).

Crystal data
Systematic absences permitted C2, Cm and C2/m as possible space groups; C2 was selected and confirmed by the subsequent structure analysis. In this space group, atoms Cu1 and O5W of the low-occupancy solvent water molecule (see below) lie on crystallographic twofold axes. Therefore, the asymmetric unit comprises, in addition to these two atoms, one of each of a complete serinate and aqua ligand and a single H atom of the solvent water molecule. The small amount of solvent water was clearly identified from the difference map. During the structure solution, and prior to the location of the water molecule, the difference map revealed two electron-density peaks close to one another, which suggested disorder of the water over two sites. However, the two positions could not be refined simultaneously and indeed, once one O atom was refined, the peak in the difference map corresponding to the 'second site' disappeared. Approximate positions for the H atoms of the aqua ligand and of the low-occupancy solvent water molecule were then obtained from difference maps and modified to provide acceptable O-H distances (   molecule could only be established by trial and error. The value of 0.10 finally chosen was such as to provide a reasonable value for the freely refined isotropic displacement parameter of the O atom (O5W). All other H atoms were placed in calculated positions, with X-H distances of 0.99 (CH 2 ), 1.00 (aliphatic CH), 0.92 (NH 2 ) or 0.84 Å (OH). The torsion angle of the OH group was also refined. All H atoms were refined, finally, with a riding model, with U iso (H) = 1.2U eq (C,N) or 1.5U eq (O).